Recently, as a small, light-weight, and high-capacity battery, non-aqueous electrolytic solution secondary batteries such as a lithium ion secondary battery have been proposed and put into practice.
A lithium ion secondary battery has lighter weight, smaller size, and higher energy as compared to other secondary batteries of the related art such as a lead battery, a nickel-cadmium battery, or a nickel-metal hydride battery. Therefore, a lithium ion secondary battery is preferably used as a power supply of a portable electronic apparatus such as a mobile phone or a laptop computer. In addition, a lithium ion secondary battery is considered as a high-output power supply for an electric vehicle, a hybrid vehicle, an electric tool, or the like. In a lithium ion secondary battery used as the high-output power supply, high-speed charge and discharge characteristics are required for an electrode active material.
Regarding the development of a lithium ion secondary battery, a rare metal-free electrode active material is considered from the viewpoint of obtaining high performance, high capacity, and low cost, and various materials are studied. Among these, an olivine-type phosphate electrode active material represented by lithium iron phosphate (LiFePO4) has attracted attention as an electrode active material which has high safety, is abundant in resource, and is inexpensive.
Among phosphate electrode active materials, lithium manganese phosphate (LiMnPO4) containing Li as an alkali metal and Mn as a transition metal or lithium cobalt phosphate (LiCoPO4) containing Co as a transition metal is known to have a theoretical capacity of about 170 mAh/g which is equivalent to that of LiFePO4. However, it has been said that lithium manganese phosphate or lithium cobalt phosphate has a problem of significantly poor utilization rate under low-rate discharge conditions as compared to LiFePO4 (for example, refer to Non-Patent Document 1).